Frequency stability and phase noise of an improved X-band cryocooled sapphire oscillator

A previously implemented cryogenic sapphire oscillator (CSO) based on a commercial cryocooler has been modified and its frequency stability and phase noise re-measured against a nominally similar liquid helium cooled CSO in the same laboratory. Assuming both contribute equally, their frequency stability and phase noise have been evaluated. We report for the oscillator a minimum Allan deviation of 3.9 × 10^-16 at 20 s, a long-term frequency drift less than 1 × 10^-14/day, and a measured single sideband phase noise of -97 dBc/Hz at 1 Hz offset from the carrier. The stated performance of the cryocooled CSO is adequate for it to be deployed at a local VLBI site for comparison against the hydrogen maser which is the current reference standard.